SmartFin

FIRE number and years to reach it

smartfin_calculate_fire_number

Read-onlyIdempotent

Works the way SmartFin's FIRE calculator does. The FIRE number today is yearly expenses divided by a safe withdrawal rate. Each year savings grow at an assumed return with the yearly savings added, while expenses (and so the FIRE number) rise with inflation; FIRE is the first year savings reach that year's FIRE number, searched up to 60 years. Also gives the savings rate and, with a traditional retirement age, the Coast FIRE number. Use for early retirement / FIRE questions: "how much do I need to retire", "when can I retire", "what's my coast FIRE number". Do not use to test how long an existing balance lasts (use smartfin_calculate_retirement_withdrawal). Returns and inflation are assumptions; excludes taxes.

Input Schema

TableJSON Schema

Name	Required	Description
`current_age`	No	For the age at FIRE and Coast FIRE. Default 30
`inflation_pct`	No	Yearly inflation in %, applied to expenses. Default 3
`annual_savings`	No	Added each year, in dollars (12 x a monthly amount). Default 0
`retirement_age`	No	Optional: a traditional retirement age, for the Coast FIRE number
`annual_expenses`	Yes	Yearly spending today, in dollars
`current_savings`	No	Invested today, in dollars. Default 0
`expected_return_pct`	No	Assumed yearly return in %. Default 7
`withdrawal_rate_pct`	No	Safe withdrawal rate in %. Default 4

Output Schema

TableJSON Schema

Name	Required	Description
`cite`	Yes	How to credit this result
`data`	Yes
`next`	Yes	Suggested follow-up calls
`links`	Yes
`how_to`	Yes	Steps for the user to see or redo this on smartfin.fyi
`source`	Yes
`summary`	Yes	Plain-language result the assistant can quote

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 29, 2026

addedOutput schema / properties / links / properties / page

Added value: +{
+  "description": "SmartFin's page for this stock, when it has one",
+  "type": "string"
+}

First observedSep 29, 2026

TDQS

A4.4/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds useful behavioral context: returns and inflation are assumptions, taxes are excluded, the search is bounded to 60 years, and the tool also returns the savings rate and Coast FIRE number when a retirement age is given. No auth or rate-limit information is needed for a read-only calculator.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is front-loaded with the purpose and algorithm, then usage examples, exclusions, and assumptions. It is appropriately sized for a complex calculator, though the opening 'Works the way SmartFin's FIRE calculator does' is slightly redundant; every other sentence earns its place.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given 8 parameters, an output schema, and read-only annotations, the description is complete: it explains what is computed, key assumptions, exclusions, and when to choose a sibling. Because an output schema exists, return values need not be detailed further.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema already documents all 8 parameters including defaults and enums. The description mentions some fields implicitly (yearly expenses, savings, return, inflation) but adds no syntax, format, or constraints beyond what the schema provides. Baseline 3 is appropriate when the schema does the heavy lifting.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('calculate FIRE number') and explains the computation in detail: yearly expenses divided by a safe withdrawal rate, savings growth with inflation, and searching up to 60 years. It also names a sibling it is not ('Do not use to test how long an existing balance lasts (use smartfin_calculate_retirement_withdrawal)'), letting an agent distinguish it without opening schemas.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly says when to use it ('Use for early retirement / FIRE questions') with three example queries, and when not to use it, naming the correct alternative (smartfin_calculate_retirement_withdrawal). The condition that selects the alternative is clear.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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